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Having a weird tach signal issue with a customers car, please help!

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GSXftw

10+ Year Contributor
381
78
Dec 16, 2009
Medford, Oregon
So let me start by saying this isnt my car, and Im really glad its not LOL. Thing is a total abortion by the PO, needs his tools confinscated. Customer brought it in with a few issues, it had a P0335B DTC (crank position sensor circuit failure)...the code is thrown immediately, before you even start it just key on. I took one look at the wiring at the sensor and replaced the CkPS, it actually had those yellow wire nuts you use on house wiring twisting wires together and shit. After replacing CkPS, no change, the code is still thrown. This is a 95 Talon TSI AWD. Now the weird part. When you get the engine to a certain position, you get a phantom tach signal. The tach shows a steady 3400 RPM, KOEO (key on engine off) and the coils fire at the 3400rpm rate, but the ECU and scanner doesnt show this signal, all is normal on that side, its getting a phantom signal on the ignitor/ECU side. Tried a different igniter, no change, I checked wavelenths of the crank/cam postion sensors, all is normal there, nothing erratic, car starts/runs, just breaks up anywhere after 3k. During this 3400 phantom signal, there is not any wavelengths coming from either the CPS, or the CkPS, you can even unplug them both and the tach stays the same, the tach wire on the igniter however shows 110hz. The engine wiring looks okay, the car does have a turbo timer and an alarm, with a shitload of skotchlocks, exposed wires, and everything else I dont wanna try and make sense out of. Any Ideas where this phantom tach signal could be coming from that could give this signal? Thanks guys
 

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110hz is what you would expect to see feedback on if both coils are firing at 3300hz.


I've not heard of anything like that before. But the ECU is the one doing the firing, and the 110hz signal at the ignitor tells me it is running at right around 3300rpm. At least it thinks it is. No need to troubleshoot the tach, it's doing what it is supposed to.


Is it the correct ECU? Does it have damage or modifications inside the ECU? Even then it would be hard to cause it. I am wondering if they don't have some weird feedback from their turbo timer or alarm wiring that is tied into something it shouldn't be. Does the alarm have a valet mode that is active, or can you completely disable the alarm or turbo timer(removing fuses etc).
 
110hz is what you would expect to see feedback on if both coils are firing at 3300hz.


I've not heard of anything like that before. But the ECU is the one doing the firing, and the 110hz signal at the ignitor tells me it is running at right around 3300rpm. At least it thinks it is. No need to troubleshoot the tach, it's doing what it is supposed to.


Is it the correct ECU? Does it have damage or modifications inside the ECU? Even then it would be hard to cause it. I am wondering if they don't have some weird feedback from their turbo timer or alarm wiring that is tied into something it shouldn't be. Does the alarm have a valet mode that is active, or can you completely disable the alarm or turbo timer(removing fuses etc).

Thank you for the reply. I havent checked the ecu yet, will probably do that next, I unplugged the turbo timer just from its harness plug, that did no change, but I need to dig a little deeper I guess. The big weird thing is since the igniter would more be outgoing from ecu, the ecu should show the same RPM, it doesnt, it shows 0 like its supposed to.
 
I meant to say 3300rpm, not 3300hz. Anyhow, I would definitely look more into the alarm. I am wondering if there isn't something from that, that could be wired incorrectly to the coils. Perhaps an RPM sensing line from the alarm, or a relay for killing the ignition?

The ECU calculates RPM from the crank/cam primarily, I believe. The feedback from the ignitor to the ECU is useful for determining misfire, but I don't think they display that anywhere. The zero rpm would rule out the cam/crank wiring, BUT the fact you have an error code there makes it seem like there is still a problem with that crank wire. Maybe that is where they have tapped in.
 
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